Drought stress effects on Pest & Disease
By Drs Glynn Percival and Jonathan Banks
2019-09-30
Trees growing within the UK’s urban landscapes and woodlands are susceptible to attack from a variety of insect pests and fungal pathogens. Climatic models predict the UK will become a hotter drier country over the next twenty years. In addition, the summer of 2018 was one of the driest on record. Therefore, the aim of this article is to review the influence of drought stress on outbreaks of tree damaging insect pests and fungal pathogens. Advice to professionals involved with tree management is also provided in order to allow proactive management.
Introduction
During the summer of 2018 the UK experienced a prolonged drought period with some areas experiencing less than 10% of average rainfall. The 2018 summer also equalled the warmest summer on record with daytime temperatures in parts of the country consistently exceeding 30oC. Of greater concern is that climatic models indicate the UK will become warmer over the coming years with mean summer temperatures estimated to increase by 1.5⁰C between 2030 and 2052, consequently, drought events are also predicted to increase. Studies have shown that changes in drought intensity and frequency have the potential to dramatically alter the impacts, populations and severity of tree-damaging insect and fungal pathogens. Increased awareness of these potential changes is important for professionals involved in urban tree management in order to anticipate the future condition of trees growing in our towns and cities as well as pro-actively develop actions to mitigate any undesirable changes.
How Drought Influences Susceptibility to Pest and Fungal Attack
Direct Effects on Insect Pest Populations and Fungal Disease Development
Fungal Pathogens
Water is essential for any fungal pathogen to complete its lifecycle since most fungi require free water or high moisture for spore dispersal, germination and infection. For example, box blight (Cylindrocladium buxicola) or Diplodia Tip Blight (Sphaeropsis sapinea; a disease of pine, fir and spruce) require precipitation and/or humidity in order to successfully produce inoculum. Likewise diseases such as rusts and powdery mildews that infect pear, box, hawthorn, rose, willow and plum have many repeated cycles during the growing season, and because of this, they tend to have a low occurrence level during periods of drought. The same negative effects on disease severity tends to occur for fine root pathogens such as Phytophthoras which are sensitive to precipitation and humidity as rates of reproduction, spread, and infection relies on flagellate zoospores swimming in free water which requires that the spaces between the soil particles are filled with water.
Within the tree itself the water content will drop during periods of drought i.e. water potentials within plant organs reduce. Eventually a water potential will be reached where leaf, root and shoot growth will no longer be possible. In contrast, however, most fungal pathogens are capable of growing at water potentials below the minimum for growth of plant cells. Botryosphaeria dothidea for example, a world-wide canker causing pathogen of deciduous and coniferous trees exhibits a strong tolerance to low water potentials, with germination and germ tube elongation occurring at extremely low water potentials. Zoosporic organisms, such as Phytophthoras, however, are generally less tolerant of low water potentials.
Insect Pests
Most tree-damaging insect pests within the UK have their main growth period during the warmer part of the year, in general, temperature elevation can increase the speed of insect development which in turn can be influenced, and often related to specific habitat and microclimate the tree is growing within. For example an oak woodland habitat would provide a cooler and damper microclimate compared to an isolated oak tree planted within a parkland where a drier and warmer microclimate exists.
In the case of species, such as the aphid and most Lepidoptera (caterpillars), higher temperatures encountered during the drought period permit faster development times, potentially allowing for additional generations within a year. In addition, many of these species could potentially expand their geographical ranges from primarily Southern to Northern Britain. In addition, research shows that higher temperatures have favoured beetle development and thus increased population size and infestation pressure are likely to occur.
Effects on Host Tree Nutritional Status
Drought is known to directly affect many components of tree “palatability” to insects. Reduced transpiration will influence the transport, production and storage of almost all macro and micro nutrients. Drought can specifically alter plant tissue concentrations of nitrogenous compounds such as amino acids and nitrate; osmolytes such as glycine betaine, proline and sugars, and allelochemicals such as cyanogenic glycosides, terpenoids, and alkaloids. Increases in these compounds tends to occur under mild or moderate drought and decreases in some compounds occur during prolonged and severe drought. Drought induced changes in the content and ratio of these compounds can have a dramatic influence on the susceptibility of a plant to insect and disease attack. For example increased damage by foliage-feeding insects can be attributed to drought-induced increased foliar nitrogen concentration. Lepidopteran larval survival for example, has been positively associated with leaf nitrogen concentration in pine. Timing and duration of water stress are important controls over insects capacity to use concentrated zones of nitrogen in plants. For example sap-feeding insects can benefit from feeding on drought-stressed plants when drought is followed by wetter periods and allow insects to benefit from drought-induced increase in plant tissue nitrogen. Armillaria spp. have been reported to be associated with tree declines following drought. One possible mechanism which may increase severity is that glucose, which increases in stressed trees, stimulates growth of Armillaria and enables it to grow in the presence of inhibitory phenols. Further research also demonstrated that levels of carbohydrates, fatty acids and amino-acids were altered in drought stressed Lawson cypress plants favouring increased growth of Armillaria mellea and A.gallica on roots. Similarly, certain amino-acids, e.g. proline, asparagine and alanine are known to stimulate hyphal growth of Entoleuca (Hypoxylon) mammata; these compounds have been found to increase in concentration in water-stressed Populus tremuloides.
A common response to drought is to increase “leaf toughness”. Leaf toughness, is also strongly associated with plant resistance against insects. Some research has reported stronger roles of physical rather than chemical traits in plant resistance to insect herbivory. During prolonged drought, leaf water content can decrease and leaf toughness and dry matter content may increase as an adaptive response. Although these physical changes are associated with reductions in herbivore (animal) feeding, drought can increase plant attractiveness to insects by altering clues used to identify hosts. Leaf yellowing, that often accompanies drought may be a spectral clue detected by insects, and the warmer temperature of drought-stressed plant tissues may be detected by insect thermal sensors. Xylem cavitation in plants caused by drought results in ultrasonic acoustic emissions that likely are detectable by some insects. Insect chemoreceptors may detect drought-induced changes in suites of plant compounds. For example, drought may induce plant production of volatile compounds and ethanol that are olfactory attractants for insects, such as bark beetles.
This article continues in part 2, looking specifically at: Urban Tree Insect Pests and Fungal Pathogens of Concern in the UK